BMR Calculator — Basal Metabolic Rate (Mifflin-St Jeor)
Calculate your Basal Metabolic Rate (BMR) — the calories your body burns at rest. Uses the accurate Mifflin-St Jeor equation for adults.
Your Basal Metabolic Rate (BMR) is the number of calories your body burns just to keep you alive — breathing, circulating blood, regulating temperature, and maintaining cells — when you are completely at rest. It is the foundation for understanding your daily calorie needs, and it is the most important starting number for anyone trying to lose, gain, or maintain weight.
Our BMR calculator uses the Mifflin-St Jeor equation, which research has shown to be the most accurate formula for the modern adult population, more reliable than older equations like Harris-Benedict. Enter your age, gender, height, and weight, and you will get your BMR in calories per day, plus an estimate of your total daily energy expenditure based on your activity level.
How BMR is calculated
The Mifflin-St Jeor equation: For men, BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5. For women, BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161.
The formula reflects three core findings about metabolism. Larger people have higher BMR (more tissue to maintain). Taller people have higher BMR at the same weight (longer body, more surface area). Older people have lower BMR (muscle mass declines with age, reducing energy needs).
The 161-calorie difference between men and women reflects average differences in body composition — men typically have more lean muscle mass, which burns more calories at rest than fat tissue.
From BMR to total daily calories
BMR is just your resting metabolic rate. To calculate Total Daily Energy Expenditure (TDEE) — what you actually burn including activity — multiply BMR by an activity factor: 1.2 for sedentary (little or no exercise), 1.375 for lightly active (1-3 days/week), 1.55 for moderately active (3-5 days/week), 1.725 for very active (6-7 days/week), and 1.9 for extra active (twice daily training or heavy physical job).
TDEE is the calorie level that maintains your current weight. To lose weight, eat 300-500 calories below TDEE for steady, sustainable loss of about 0.5-1 pound per week. To gain weight, eat 300-500 calories above TDEE. Aggressive deficits or surpluses (1000+ calories) cause more rapid changes but are harder to sustain and can backfire.
Why metabolism slows with age
BMR declines about 1-2% per decade after age 20, primarily due to gradual loss of muscle mass (sarcopenia). A 25-year-old man weighing 80 kg might burn around 1,800 BMR calories. The same man at 65 burns around 1,600 — a 200 calorie reduction without any change in weight.
This is why people often gain weight in middle age without changing their diet — their bodies need fewer calories than before. Strength training is the most effective way to slow this decline because it preserves and builds muscle, keeping metabolic rate higher.
How to use this calculator
- Choose your gender (BMR formulas differ for men and women).
- Enter your age in years.
- Enter your height (in centimeters or feet/inches).
- Enter your current weight (in kilograms or pounds).
- Optionally select your activity level for a TDEE estimate.
- Click Calculate to see your BMR and daily calorie needs.
Worked example: 35-year-old woman, 65 kg, 165 cm
BMR = (10 × 65) + (6.25 × 165) − (5 × 35) − 161 = 650 + 1,031 − 175 − 161 = 1,345 calories per day.
If she is moderately active (gym 3-5 days/week), her TDEE is 1,345 × 1.55 = approximately 2,085 calories per day to maintain weight.
To lose about 1 pound per week, she would target around 1,585 calories per day (500 below TDEE). To gain muscle, she could target 2,385 calories per day with strength training.
Frequently asked questions
Is BMR the same as resting metabolic rate (RMR)?
They are very close but slightly different. BMR is measured in strict lab conditions (12 hours fasted, complete rest). RMR is measured under more practical conditions and is typically 5-10% higher. Most online calculators give an RMR-equivalent value.
Can I increase my BMR?
Yes. Strength training builds muscle, which raises BMR (each pound of muscle burns about 6 calories at rest versus 2 for fat). Eating enough protein supports muscle. Avoiding extreme calorie restriction prevents metabolic slowdown.
Why is my BMR different from someone the same weight?
Body composition matters. Two people at the same weight can have very different BMRs based on their muscle-to-fat ratio. Age, gender, and height also play roles.
Should I eat less than my BMR to lose weight?
Generally no. Eating below BMR is hard to sustain, can slow metabolism further, and may cause muscle loss. Aim for a moderate deficit from TDEE (300-500 calories below maintenance) for steady, sustainable loss.